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Sodium hypochlorite (method electrolysis) and Chlorine dioxide

Summary: Disinfection of drip watering or irrigation systems in horticulture is a necessary activity. For good results, multiple active components are suitable. Ci-Ro used as Chlorine-containing biocides Sodium hypochlorite and Calcium hypochlorite as well as a new system based on Chlorine dioxide.

About Sodium hypochlorite is already well known. In particular, the pH setting is important for a good result.

For Chlorine dioxide is developed a two-component system that is simple, safe and can run without expensive equipment disinfection with Chlorine dioxide.

We offer a complete outline that shows the technique and the chemistry of both systems.

Sodium hypochlorite

In the market are several systems available that are based on the electrolysis of Chlorine - containing chloride salts (common salt, potassium salt, etc.), the so-called Aquanox and the ECA machines.

Hereby, on location is made Hypochlorite (Sodium hypochlorite) from salt ​​or ​​caustic potash (Potassium hypochlorite). Sodium hypochlorite is used in various concentrations.

It is also known as Chlorine bleach and is used in a variety of applications such as biocide against bacteria, fungi and algae.

The 12.5% ​​quality is only used in industrial plants. For domestic use it is always used with in a diluted version. The chemical formula of Sodium hypochlorite is NaOCl.

Dissolved in water gives this as a final product HOCl and NaOH.

When the Sodium hypochlorite solution is added to the water it splits, depending on the pH, in HOCl and OCl -. 

The addition of NaOCl to water results in Hypochlorous acid:

In comparison: NaOCl + H2O ⇔ HOCl + NaOH

Hypochlorous acid is a weak acid that partial dissociation undergoes according to the equation: NaOCl + H 2O Û HOCl + NaOH

Depending on the pH (acidity) is shifting the equation to the left or to the right (HOCl or OCl -)

In solutions with pH lower than 4, present is a mixture of HOCl and Cl 2. Between pH 4 and 11; a mixture of HOCl and OCl -, and at pH 7.5, the ratio of OCl and HOCl is equal to 1.

In strongly alkaline solutions (high pH), the balance of the equation is entirely to the left, above pH 11, all of the Chlorine is present as OCl -.

Hypochlorous acid (HOCl) is a highly reactive compound, and thereby a stronger disinfectant than hypochlorite. Above pH 11 the biocidal effect of the solution decreases.

In the Chlorine chemistry, various terms are used to express the concentration of active Chlorine. The content of free Chlorine is the sum of the contents dissolved as Cl 2, HOCl and OCl  -. (At pH > 4, this is only HOCl / OCl -).

The concentration of free Chlorine is expressed as mg Cl 2 / L.

Total Chlorine includes free Chlorine and in addition also the fraction of bound Chlorine (chloramines, but also other oxidation products).

Total and combined Chlorine are expressed as mg Cl 2 / L.

The formation of bound Chlorine is an important disadvantage of the use of Chlorine bleaching solutions in systems where hydrocarbons and ammonia compounds are present. The reaction products are often poorly degradable and harmful to the environment.

Sodium hypochlorite in water.

Depending on the pH you get HOCl and OC l-, much HOCl at a low pH and at high pH a little HOCl.

This means that at high pH the dosing of Sodium hypochlorite must be high just to get a little active Chlorine. If the water is, for one reason or another become acid, than all of this Chlorine will be working and create a dosage problem. (Overdose)

Vice versa the same is also true; Chlorine dosing at a low pH is effective at a low concentration but at a high pH it is no longer effective against bacteria.

HOCl is the most active component that will react directly with bacteria, fungi, and algae as well as with hydrocarbons and ammonia compounds. When it is sufficiently dosed remains an excess of HOCl.

This can be measured with the free Chlorine test.

Sodium hypochlorite is available as a turnkey solution that can be used directly for disinfection.

Disadvantage of Sodium hypochlorite is that the activity in the tank is a decreasing function of time. A solution of Sodium hypochlorite to 150 g/L of active Chlorine, which is stored at 15° C in the dark, loses in less than 3 months 1/6 of the concentration of active substance. Under bad storage conditions about half is lost in that time.

Preparation by electrolysis (kitchen) salt.

In a solution of common salt (Sodium chloride) an anode and cathode is used to carry out electrolysis. The electrodes are connected to a power supply.

The cathode is negatively charged, the anode positive.

Sodium chloride is dissolved in water and splits into Na + and Cl -, attracts negative to positive and vice versa.

Sodium+ moves to the cathode -, and there take up an electron, after which the element Sodium is formed.

Sodium in water will react immediately and is converted to Sodium oxide, and finally to Sodium hydroxide solution. (NaOH)

Chloride- goes to the anode +, release an electron, and Chlorine gas (Cl 2) is formed. Chlorine in water dissolves and becomes Hypochlorous acid (HClO -).

Sodium hydroxide and Hypochlorous acid in solution form Sodium hypochlorite.

Advantages of this method:

·        No transport of containers Sodium hypochlorite.

·        Fresh preparation, according to need, is possible.

Disadvantages:

·        Efficiency, side reactions, electrolysis of water produces hydrogen and oxygen, forming highly explosive mixture.

·        Keep the concentration of Sodium chloride

·        Energy consumption of the installation.

·        Maintenance of the installation.

·        High purchase price of the plant.

 Contrary, to what is claimed; at low salt concentrations there is no Chlorine dioxide release.

 Application Horticulture:

The electrolysis technique is mainly applied in swimming pools, but there are also systems for greenhouses.

In horticulture Potassium chloride is used as a salt. The electrolysis of Potassium chloride is equal to that of Sodium chloride, with the proviso that there are, of course, no NaOH but KOH is formed.

You will get a Potassium hypochlorite solution instead of Sodium hypochlorite with almost identical characteristics.

In horticulture Potassium is a fertilizer and high concentrations of Sodium are undesirable.

Additional flushing may reduce the accumulation of the Sodium Concentration.

The fact remains that it is a very expensive installation for making a disinfectant.

Sodium and Potassium hypochlorite you can buy at any street corner for a low price.

Ci-Ro can provide a cheap solution and can supply Calcium hypochlorite.

It is a solid, contains no Sodium and no special equipment is needed.

 Chlorine Dioxide.

The chemistry: Chlorine dioxide is formed by the reaction between a Chlorite and an acid.

Equation: 5 NaClO 2 + 4 HCl 4ClO 2 + 5NaCl + 2H 2O

The reaction is highly exothermic (a lot of heat development), and an explosive gas is formed which is dissolved in water.

Chlorine dioxide has a very strong and fast reaction with bacteria, fungi and algae. The very short kill time is in the order of milliseconds. An additional advantage is that it is almost exclusively reacts with these organisms and little to no side reactions occur. So, no chlorinated hydrocarbons and chloramines are formed. Reactions with dissolved iron and manganese are well known.

Chlorine dioxide is widely used as a disinfectant in the paper industry, water treatment, food processing and in cooling towers.

Advantages:

·        Fast effective killing of bacteria, fungi, algae and viruses in a low concentration which is not influenced by the pH.

·        There is no reaction with trace elements such as Iron chelate, etc., and the other fertilizers in the water.

Disadvantages:

·        No addition of Oxygen is possible.

·        Risk of explosion in the production of Chlorine dioxide when used 

      improperly or with wrong equipment.

·        Reasonably fast degassing when the water is aerated. 

      A low exposure limit in air (0.3 PPM) requires careful work procedures.

Preparation:

The dosing system that makes Chlorine dioxide is fully adapted to these risks and consists of pumps for a Sodium Chlorite, Hydrochloric acid and water. Water is for dilution water and for the reactor.

A PLC computer controls the installation. The desired production of Chlorine dioxide concentration (Grams per hour) is entered after which the various pumps are controlled in order to make this quantity.

Use in horticulture:

Accurate dosing control by means of a PLC.

Dosing is provided with required safety devices.

Attached is a picture of test equipment in which in one reactor is filled with water and in a glass reactor tube inside the pure products are mixed together. In the reactor is visible a brown liquid containing a quantity of gas.

This liquid / gas mixture is directly absorbed in water; thereby preventing the equipment would explode.


For each plant protection product or biocide, the user must adhere to the conditions imposed by the admission in the country. Use plant protection products and Biocides safely. Before use, read the label and the MSDS. Possibility of sale is subject to registration requirements in the different countries.